The present invention relates to a valve timing control apparatus for an internal combustion engine comprising a variable valve timing control mechanism capable of optimizing an open or close timing of at least one of intake and exhaust valves in accordance with engine operating conditions, and an air-fuel control device capable of varying an air-fuel ratio of gas mixture introduced into a combustion chamber of the internal combustion engine.
Unexamined Japanese Patent Application No. Kokai 7-91280, published in 1995, discloses a valve timing control apparatus for an internal combustion engine including a hydraulically controlled variable valve timing control mechanism wherein a control amount is adjustable in accordance with oil nature to improve drivability. Japanese Patent No. 7-68921, published in 1995, discloses another hydraulically controlled variable valve timing control mechanism wherein a control amount is variable to stabilize an idle operating condition of an internal combustion engine.
This kind of valve timing control apparatus may be incorporated into a so-called lean burn type internal combustion engine equipped with an air-fuel ratio control device which flexibly varies the air-fuel ratio of gas mixture introduced into a combustion chamber. However, undesirable change of the valve timing of the intake or exhaust valve leads to unstable combustion during a lean bum control mode. For example, advancing the valve timing of an intake valve will increase an overlap between the opening period of an exhaust valve and the opening period of the intake valve. The increased overlap between the intake and exhaust valve opening periods possibly increases a residual exhaust gas amount remaining in the combustion chamber. In other words, a so-called internal EGR amount is increased, while worsening the combustion or causing a misfire. This greatly worsens the emission and the drivability.